Typically, solid oxide fuel cells (SOFC) employ an electrolyte made up of an ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (MEA). The electrolyte electrode assembly is sandwiched between a pair of separators (bipolar plates). In use, generally, a predetermined number of separators and electrolyte electrode assemblies are stacked together to form a fuel cell stack.
In the fuel cell, in order to supply a fuel gas such as a hydrogen-containing gas, and an oxygen-containing gas such as air to the anode and the cathode of the electrolyte electrode assembly, respectively, a fuel gas channel and an oxygen-containing gas channel are formed along surfaces of the separators.
For example, as shown in FIG. 9, a flat plate stacked type of fuel cell, as disclosed in Japanese Laid-Open Patent Publication No. 2006-120589, includes a separator 1a stacked on a power generation cell. The separator 1a includes left and right manifold parts 2a, a central part 3a where the power generation cell is provided, and connector parts 4a that interconnect the left and right manifold parts 2a and the central part 3a. The connector parts 4a are flexible.
Further, as shown in FIG. 10, a fuel cell stack, as disclosed in Japanese Laid-Open Patent Publication No. 2007-317594, is formed by alternately stacking cells 1b and separators. Each of the separators includes a cathode plate member 2b and an anode plate member 3b. 
The separator has an internal manifold structure including a fuel electrode supply manifold 4b for supplying a fuel gas to a fuel electrode, an air electrode supply manifold 5b for supplying an oxygen-containing gas to an air electrode, and a fuel gas discharge manifold 6b for discharging unconsumed fuel gas as well as consumed fuel gas as an exhaust gas.
Two cells 1b are placed in cell holders 7b. The fuel electrode supply manifold 4b, the air electrode supply manifold 5b, and the fuel gas discharge manifold 6b are interposed between the cell holders 7b. 
Further, as shown in FIG. 11, in a separator, as disclosed in Japanese Laid-Open Patent Publication No. 2007-329063, a fuel supply manifold 1c for supplying a fuel gas from the outside, a fuel discharge manifold 2c for discharging the fuel gas to the outside, and an oxygen-containing gas supply manifold 3c for supplying an oxygen-containing gas from the outside are connected to a center plate 7c through pipes 4c, 5c and 6c, respectively. In the separator, fuel gas is supplied and discharged, and oxygen-containing gas is supplied through the pipes 4c, 5c and 6c. 